Various techniques can be used to grow materials used in semiconductor devices. One popular technique is molecular beam epitaxy. Generally, in a molecular beam epitaxy deposition process, thin films of material are deposited onto a substrate by directing molecular or atomic beams to a deposition region where a substrate is positioned, typically by a substrate manipulator capable of heating the substrate. Deposited atoms and molecules migrate to energetically preferred lattice positions on the heated substrate, yielding film growth of high crystalline quality and purity, and optimum thickness uniformity. Molecular beam epitaxy is widely used in compound semiconductor research and in the semiconductor device fabrication industry, for thin-film deposition of semiconductors, oxides, metals and insulating layers.
Conventional molecular beam epitaxy growth chambers typically use a liquid nitrogen filled cryogenically cooled shroud (cryoshroud or cryopanel) that substantially surrounds and encloses the active growth region. The cryoshroud functions to pump the growth chamber, particularly the growth region, by condensing residual species, especially volatile high vapor pressure species, not removed or trapped by the primary vacuum pumping system. The cryoshroud can also enhance the thermal stability and temperature control of critical growth reactor components such as effusion sources and can condense and trap source material emitted from the effusion cells but not incorporated into the growing film.
One challenge associated with certain molecular beam epitaxy processes, such as those for growth of nitride and oxide materials relates to the significant amount of gas that needs to be pumped away to maintain the desired vacuum level for the growth environment. In a typical molecular beam epitaxy deposition system, gas can be pumped by the cryopanel of the growth reactor. However, because gases used for growth of materials such as nitrides and oxides often have a generally high vapor pressure, such gases are susceptible to being reevaporated from the cryopanel. For example, radiant heat can impinge upon different surface portions of the cryopanel or adjacent chamber structure at different times during a typical deposition process because of the opening and closing of shutters on effusion sources or other heat sources or instruments. This can cause a surface portion of the cryopanel to vary in temperature during a deposition process which can cause gas to be pumped when the surface portion is cold enough and reevaporated when the surface portion increases in temperature.